There are a variety of processes in the art which utilize the contacting of gaseous and liquid phases. A common process involves the introduction of the gaseous phase in the form of bubbles which make contact with the liquid phase as they pass through that phase. This common process is used in a number of applications and for a number of purposes. A hot gas may be bubbled through a liquid to transfer heat from the gaseous phase to the liquid phase or the gas may be at a lower temperature to absorb heat from the liquid phase and into the gaseous phase.
There are conventional processes for the mass transfer of a material from the liquid to gaseous phase. Boiling, sparging and distillation.
There are three effects achieved by boiling: (i) provision of the latent heat of vaporization, (ii) raising the temperature of the liquid so that the temperature of the vapour that is in equilibrium rises, hence raising the saturation pressure of water vapor or the absolute humidity achievable, (iii) increasing the gas-liquid interfacial area so as to increase the rate of evaporation. So if the aim is vaporization, most of the applied heat is actually used to raise the water temperature, rather than to “pay” for the latent heat of vaporization and to raise the absolute level of humidity achievable. This is an unavoidable consequence of equilibrium.
When a hot bubble is injected into a cold liquid, there is a non-equilibrium driving force for both heat and mass transfer, but also, depending on the compositions of the phases, for phase change by evaporation or condensation, which is often referred to as a flash. Conventional distillation (batch or continuous columns and fractional distillation) heat the liquid in reboilers and pre-heat the liquid feed stream, so if there are microbubble clouds generated, they are hot bubbles in a hot liquid.
In addition to sensible heat transfer many processes involve mass transfer from one phase to the other or between both phases to each other. One such process of mass transfer is sparging. Typically with sparging a chemically inert gaseous phase is introduced to a liquid phase to remove a material such as a dissolved gas from the liquid e.g. removal of hydrogen or oxygen. In other variants that gas removes a low-boiling volatile component of the liquid phase. Sparging may be carried out in the absence of applied heat although in many examples either the gaseous or liquid phases or both may be heated prior to or during contact with each other. Another process uses the gaseous phase to introduce a material into the liquid phase. Often this is dissolution of a gas e.g. oxygen into the liquid phase. In some examples the totality of the introduced gas is dissolved in the liquid phase and in other variants the gas to be dissolved is in admixture with a carrier gas.
In other processes the gaseous and liquid phases contain materials for bi-molecular or other more complex reactions. The resulting bi-molecular reaction may occur mostly on the bulk gaseous phase of the bulk liquid phase with the resulting products either passing into the liquid phase or the gaseous phase or both phases. The resulting by products or waste may also pass into the liquid phase or the gaseous phase or both phases. In these reactions a catalyst for the reaction may be supplied through either or both phases.
Other processes result in bi-molecular reactions at the interface of the gaseous phase and the liquid phase and not in the bulk phase of either, where one component of the reaction is brought to this interface in the gas and the second is brought to this interface in the liquid. The resulting products either passing into the liquid phase or the gaseous phase or both phases. The resulting by products or waste may also pass into the liquid phase or the gaseous phase or both phases. In these interfacial reactions a catalyst for the reaction may be supplied through either or both phases.
In other processes a catalyst may be present in the gaseous or liquid phase and is transferred into the other phase to catalyse a reaction in that phase, with either products or waste materials being removed from that phase and into the other phase.
Microbubbles are known and have been utilised in a number of applications. Until recently, generating clouds of microbubbles was a relatively expensive proposition, with the smallest bubbles requiring high energy density from either the saturation-nucleation mechanism or Venturi effect. Due to the expense of processing with microbubbles, exploration of the acceleration effects of microbubbles for physicochemical processes are largely unstudied, particularly those that are combined effects.
For example in published international patent application WO2008/053174, there is described a method and apparatus for the generation of microbubbles. Various processes for gas dissolution or sparging and other applications are discussed.
In published U.S. Pat. No. 5,422,044, there is described a gas injection and heating device and method for the bubbling of a gas into a body of hot liquid to be interacted with the gas. The device comprises an elongate heat exchange gas container, designed to be immersed in the hot liquid to pre-heat the gas in situ by heat exchange with the liquid. Cold gas is supplied to the elongate gas container, circulated there through to become heated to the liquid temperature, and then released from a nozzle into the depth of the liquid in the form of small bubbles of hot gas having a large liquid interfacial mass transfer area.
In published U.S. Pat. No. 5,030,362 there is described a process for stripping liquid systems and a sparger system wherein Undesirable materials, such as unreacted raw materials and by-products, are stripped from liquid systems by delivering a compressed, inert gas through the pores of a sintered porous sparger element and into the liquid system in the form of very small gas micro bubbles.
In published U.S. Pat. No. 5,202,032 there is described a method of removing and recovering hydrocarbons from hydrocarbon/water mixtures in which the hydrocarbons are stripped from non-flowing hydrocarbon/water mixtures by a batch procedure by stripping with a stream of inert gas, such as air, introduced into the mixture under pressure, whilst contained in a tank and preferably heated. Preferably two tanks are used, the one being stripped whilst the other is filled.
In published United States Patent Application No. 2006/0102007, there is described a cold method of heated distillation by manipulating bubbles, and code distillate condensation. The continuous method introduces counter-current gas bubbles to a solution under vacuum at cold temperatures, using passive bubble manipulation. The approach accomplishes volatile evaporation at temperatures too low for thermal damage to occur, scrubs distilland mist from evaporated distillate, and condenses distillate by adding little or no heat. The method operates between freezing and ambient temperatures, but primarily near freezing, thus reducing energy consumption, and completely avoiding common thermal damage to delicate aroma, flavor, color, and nutritional distallate constituents that are characteristic of conventional aroma or essence extraction, food or drink concentrations, and chemical separation processes.
In published U.S. Pat. No. 5,211,856 there is described a method for low vacuum oil/water mixture liquid separation and an oil purification device for oil/water separation. Fully diffused purified gas is introduced into an oil/water mixture liquid in a low vacuum container, enabling the liquid to produce concentrated micro fine gas bubbles, enabling the liquid to be in a state of gas/liquid two-phase mixture. This greatly increases the surface area of the oil/water mixture liquid, speeding up the oil/water separation. This invention provides an oil/water separation rate ten times higher than that of the conventional method. This invention is not only suitable for the purification of new oil, but is adequate in the recovery, regeneration and purification of various water lubrication oils, hydraulic oils, and transformer oils.
In Japanese Laid Open Patent Application 2007-54722 (Hitachi Brand Technology Co. Ltd), there is described a liquid concentration method and apparatus in which concentration proceeds using evaporation from a liquid surface characterised by the provision of micro air bubbles to liquid in a flow channel and its subsequent heating, the evaporation of volatile components into the said micro air bubbles flowing in said channel, subsequent gas-liquid separation and, the liquid obtained from gas-liquid separation being the concentrated product to be collected.
Despite extensive research in the area of processes for mass transfer involving gaseous and liquid phases there are still areas that are problematic. In particular is that conundrum associated with mass transfer whilst seeking to avoid or limit heat transfer. The present state of the art is unable to address this problem. Under current understanding in order to secure adequate levels of mass transfer between a liquid and gaseous interface long contact times are required and the efficiency of this contact has been enhanced by utilising high surface area bubbles such as microbubble. However, what is good for mass transfer is also good for heat transfer and both usually go hand in hand with conventional gaseous and liquid contact processes. In fact in many situations it is expected that sensible heat transfer will prevail over mass transfer. Thus the present processes are problematic when seeking to avoid sensible heat transfer as in the case for example of the removal of volatile materials from heat sensitive mixtures. Such separations are either impossible or require very complicated and expensive separation protocols that may introduce other problems such as contamination.